Sprawling locomotion in the lizard Sceloporus clarkii: the effects of speed on gait, hindlimb kinematics, and axial bending during walking

1. What they did

Lizards are thought to push themselves forward mainly with their hind legs, but until this study nobody had measured how the hind leg moves over several strides by the same animal, or looked at leg and body movements together. The authors asked how a lizard with a sprawling posture (legs held out to the side) changes its leg and body movements to go faster.

They used Sceloporus clarkii, a lizard thought to move in a generalized sprawling way and with a body shape typical of its family (Phrynosomatidae).

• Filming. Lizards collected near Tucson, Arizona, were filmed on a treadmill with high-speed video (200 fields per second), from the side and, with mirrors, from above. Painted dots marked the backbone, hip, knee, ankle and the tip of the longest (fourth) toe.
• Speeds. Three preset speeds covered a threefold range, from about 1 to 3 km/h (0.27, 0.48 and 0.83 m per second). They used 22 strides of the right hind leg in which the lizard exactly matched the treadmill speed: 2 strides from one lizard at the slowest speed and 5 strides each from two other lizards at the two faster speeds.
• Measurements. From the video, they measured angles of the pelvis, hip, knee and ankle, the timing of each movement within the stride, the timing of all four footfalls (gait), how far the knee and toe swung sideways (a sign of how sprawled the leg was), and the side-to-side bending of the body from snout to tail.
• Analysis. They tested which measurements changed with speed using regression.

2. What they found

Gait

• At all three speeds the lizards used a trot: diagonally opposite front and hind feet moved together. Their gait went from a moderate walking trot to a fast walking trot, and at the fastest speed was close to becoming a run.
• From the slowest to the fastest speed, stride, stance and swing durations fell by about 40%. Stride frequency rose from 3.4 to 5.8 strides per second, and stride length from 80 to 143 mm.
• The feet spent a smaller share of each stride on the ground as speed rose (hind feet 63.9% to 52.4%; front feet 60.7% to 47.9%), and the hind feet were always on the ground slightly longer than the front feet.

Leg movements

• None of the joint angles or ranges of movement changed with speed.
• In real time, 13 of 17 timing measures got faster as speed increased, as expected.
• When timing was scaled to stride length, only three measures changed: the femur (thigh bone) was pulled back faster, reaching full retraction earlier (from about 64% to 36% of the stride, about 44% faster), and the knee bent to its minimum sooner during the stance.
• The ankle movements that push the foot off the ground did not change with speed.
• The sideways swing of the knee and toe did not change, so the lizards kept the same sprawling posture and did not stand more upright when going faster.

Pelvis and body bending

• Maximum pelvic rotation stayed between 16 and 25 degrees at all speeds and peaked just before the foot lifted.
• The body bent in a standing wave with two still points (nodes): one at the shoulder girdle and one just in front of the pelvis. These did not shift with speed.
• Only the snout’s side-to-side movement changed, decreasing as speed increased (from 20.8 to 11.0 mm), so the head was held more steadily forward.

3. What they concluded

• Over this range of walking speeds, the lizards went faster almost entirely by pulling the thigh back faster, while everything else stayed the same.
• A similar pattern in a salamander suggests this may be a general way that sprawling animals speed up at walking speeds.
• Of three ideas about how lizards run faster (longer strides through more body bending, switching to a travelling wave of body bending, or more force from the legs), their results support only the third, and point to femoral retraction as the main mechanism.
• This supports the idea that leg length, especially femur length, and the size of the caudifemoralis muscle (the main muscle that pulls the thigh back) matter for lizard running performance, and that this muscle was important in the evolution of walking on land and of walking on two legs.
• The speeds studied were only about 14% to 44% of the lizards’ maximum sprint speed. At higher speeds, body bending, stride length, push-off force or posture may change too, and this still needs study.
• Sprawling and upright walking may work more alike than has been thought.

6. Other technologies, skills or innovations

• NAC HSV-400 high-speed video system at 200 fields per second, under strobe lights, with mirrors to film side and top views at once.
• Motorized canvas treadmill with a background marked every 10 cm.
• Measurement TV software for digitizing the painted landmarks frame by frame.